This invention relates to an ultrasonic probe to be used in an ultrasonic diagnostic apparatus for instance, and also to a method for manufacturing a monocrystal material suited for use as a piezoelectric element in an ultrasonic probe.
An ultrasonic probe is a device which is mainly constituted by a piezoelectric element and adapted to be used for imaging the inner structure of an object. The imaging of the inner structure of the object can be performed by emitting ultrasonic wave to the object and then receiving reflected waves from various interfaces of the object, the interfaces of which being different in acoustic impedance from each other. Examples of an ultrasonic imaging apparatus adopting such an ultrasonic probe include a medical diagnostic apparatus for examining the interior of human body, and an inspection apparatus for detecting a flaw in the interior of welded metallic portion.
With respect to a medical diagnostic apparatus, the diagnostic capability has been greatly improved due to the development of the CFM method (Color Flow Mapping method) enabling the velocity of bloodstream to be color-displayed in addition to the display of the tomogram (B-mode image) of human body. According to this CFM method, an ultrasonic wave is emitted to the heart, liver or carotid arteries, and the velocity of bloodstream is two-dimensionally color-displayed by making the most of the Doppler shift derived from the bloodstream. This CFM method is now utilized in the diagnosis of various kinds of organs of human body such as the womb, liver, spleen etc. It is also considered as being possible to diagnose thrombus of blood vessel in the heart for instance by making use of the CFM method while contacting an ultrasonic probe onto the surface of human body. Therefore, studies are now continued for developing a more sensitive apparatus of this kind.
Meanwhile, in the case of the B-mode image, even a small lesion or void due to a physical change is desired to be clearly observed down to a deep part thereof. Therefore, it is desired to obtain an image of high resolution in high sensitivity. On the other hand, in the case of Doppler mode for obtaining a CFM image, a reflection echo from minute blood cell having a diameter of several microns is utilized, so that the level of signal to be obtained is lower than that in the case of the aforementioned B-mode. Therefore, the requirements for high sensitivity in the Doppler mode are more severe as compared with the B-mode.
Conventionally, various improvements have been made on the ultrasonic probe itself or on the diagnostic apparatus for achieving a high sensitivity. For example, in the case of the B-mode, since the influence of a piezoelectric element functioning as an ultrasonic transmitting-receiving element upon the performance of ultrasonic probe can not be disregarded, a material which is high in electromechanical coupling coefficient and in dielectric constant has been employed. Since a material having a large dielectric constant can be easily matched with transmit-receive circuit, any loss due to the capacitance of cable or apparatus can be minimized. As a material meeting these requirements, a lead zirconate titanate-based piezoelectric ceramics (PZT) has been predominantly employed.
As for the ultrasonic probe, an array type ultrasonic probe where about 10 to 200 pieces of strip-shaped piezoelectric elements are arranged has been predominantly employed. The number of piezoelectric elements in the ultrasonic probe tends to increase as a demand for an ultrasonic probe of higher resolution is increasing. However, in view of avoiding the contact failure of the ultrasonic probe onto a living body, the aperture of ultrasonic wave-emitting face can not be enlarged over a predetermined limitation. As a result, as the number of piezoelectric elements in an ultrasonic probe increases, the size of individual piezoelectric element is required to be minimized, thus giving rise to a problem that piezoelectric elements can be hardly matched with the transmit-receive circuit.
There have been proposed various methods for solving the aforementioned problems. For example, U.S. Pat. No. 4,958,327 discloses a laminate structure of piezoelectric elements, and German Patent No. 3,729,731 A1 discloses the employment of an impedance converter.
However, both of them are not necessarily satisfactory for solving the aforementioned problems. For example, if piezoelectric elements are formed into a laminate structure, the transmitting sensitivity may be improved as the number of layers constituting the laminate is increased, but the receiving sensitivity is deteriorated on the contrary. Therefore, the applicability of the laminate structure of piezoelectric elements is limited to a special applications such as where a vibrator is extraordinary smaller or where the cable is extraordinary longer. On the other hand, if an impedance converter such as an emitter follower is employed, it will lead to an enlargement of the ultrasonic probe, and at the same time the frequency of the ultrasonic probe may be restricted to a narrow-band due to the frequency characteristics inherent to the impedance converter.
Moreover, the conventional PZT type ceramics tends to indicate a smaller electromechanical coupling coefficient as the relative dielectric constant thereof exceeds over 4,000, thus giving rise to another problem that the sensitivity thereof may be deteriorated. There are also known other kinds of piezoelectric materials, e.g. monocrystalline materials such as lithium niobate; ceramics such as lead titanate and lead metaniobate; and polymer materials such as polyvinylidene fluoride or a copolymer thereof. These piezoelectric materials however are too small in dielectric constant and in electromechanical coupling coefficient to put them to practical use.
On the other hand, it has been also proposed to employ a composite piezoelectric element (or a composite piezoelectric body) wherein a columnar or powdery piezoelectric material is buried in a resin as disclosed in Japanese Patent Publication Shou/54-19151; Japanese Patent Unexamined Publication Shou/60-97800; Japanese Patent Unexamined Publication Shou/61-53562; and Japanese Patent Unexamined Publication Shou/61-109400. Methods for manufacturing such a composite piezoelectric element are disclosed for example in Japanese Patent Unexamined Publication Shou/57-45290; Japanese Patent Unexamined Publication Shou/58-21883; Japanese Patent Unexamined Publication Shou/60-54600; Japanese Patent Unexamined Publication Shou/60-85699; Japanese Patent Unexamined Publication Shou/62-122499; and Japanese Patent Unexamined Publication Shou/62-131700.
The composite piezoelectric element is advantageous over the single piezoelectric element in that the acoustic impedance thereof is closer to that of living body, since the acoustic impedance of the composite piezoelectric element is smaller than that of the single piezoelectric element. Moreover, in the case of 1-3 type or 2-2 type composite piezoelectric body among others, the electromechanical coupling coefficient can be further improved as compared with a composite piezoelectric element of thin plate type. For the manufacture of the composite piezoelectric element, a PZT-type piezoelectric ceramics has been predominantly employed in view of high dielectric constant and high electromechanical coupling coefficient k.sub.33 thereof.
However, as a matter of fact, the composite piezoelectric element is accompanied with a problem that since it involves the inclusion of a resin, the dielectric constant may be caused to be deteriorated and the improvement on electromechanical coupling coefficient is not so prominent as compared with the lowering of the dielectric constant. Therefore, the composite piezoelectric element composed of a monocrystalline material and a resin is useful only for a single-type mechanical probe having a large element surface or an annular array, and is scarcely used for the prevailing types of the device such as a phased array, a convex array or a linear array.
As explained above, there have been proposed various means for obtaining an ultrasonic probe of high sensitivity, such as a method of employing a piezoelectric ceramics of high dielectric constant, e.g. a lead zirconate titanate; a method of interposing an impedance converter between a vibrator and a cable; or a method of forming a piezoelectric material into a laminate structure. However, any of these proposals have accompanied with the aforementioned problems.
As for other kinds of piezoelectric materials, the dielectric constant and electromechanical coupling coefficient thereof are so small that it is difficult with the employment of them to achieve a sufficient improvement on the sensitivity of ultrasonic probe. Furthermore, in the case of a composite structure composed of a piezoelectric material and a resin, an improvement on electromechanical coupling coefficient is not so prominent as compared with the lowering of the dielectric constant, so that such a composite structure is not employed for a general-purpose ultrasonic probe.
Recently, a piezoelectric monocrystal consisting of a binary system represented by a general formula Pb{(B1B2).sub.1-x Ti.sub.x }O.sub.3 is attracting attentions as a new candidate for the piezoelectric material. In this formula, B1 denotes at least one element selected from the group consisting of Zn, Mg, Ni, Sc, In and Yb; B2 is at least one element selected from the group consisting of Nb and Ta; and the content of lead titanate is in the range of 0 to 55 mol. %. In this perovskite compound, 10 mol. % or less of lead may be substituted by at least one element selected from the group consisting of Ba, Sr, Ca and La.
When this monocrystalline material is employed for the manufacture of an ultrasonic probe to be driven with a low frequency, the material can be formed into a thin monocrystal so that it can be cut in high precision into a rectangular bar vibrator even with a thin blade. As a result, the yield with respect to high working precision can be improved, thereby making it possible to suppress a decrease of sensitivity and an increase of side lobe level.
However, a solid solution type monocrystal such as Pb[(B1B2).sub.1-x Ti.sub.x ]O.sub.3 has drawbacks that the interior of the monocrystal is more likely to be damaged by inclusion of flux, cracks or flaws, so that the cutting of the monocrystal for preparing a vibrator for an ultrasonic probe has to be carried out while avoiding the aforementioned defective regions of the monocrystal. A standard size demanded for a vibrator for a cardiac probe for use in an ultrasonic diagnostic apparatus is 15 mm.times.15 mm.times.0.4 mm. However, since the cutting-out of the vibrator from the monocrystal is performed while avoiding a defective region affected for instance by an inclusion, the yield of the vibrator of this size is actually extremely low.
Furthermore, there is another problem that when the vibrator is subjected to a polarization treatment by applying a direct current thereto after electrodes are formed on the vibrator, cracks may be generated in the vibrator. The cracking of the vibrator may be generated at ratio of up to 50% in worst case, thus badly hindering the mass production of the vibrator.